Killer Asteroids: A Real But Remote Risk?

Events such as the impact implicated in the dinosaur extinction happen on the order of once every 100 million years. Smaller objects collide with Earth with greater frequency. Asteroids large enough to cause ocean-wide tsunamis, for example, happen once every 63,000 years.

In 1998 NASA accepted the responsibility of compiling a catalog of at least 90 percent of NEOs of 1 kilometer (0.62 miles) in diameter or greater and to assess the probability that any of them will impact Earth. Such events are believed to happen on the order of about once every 1 million years.

To date the NASA initiative, known as Spaceguard, has identified 585 objects of 1 kilometer or greater. Most of them have no chance of impact and those that do have only a very low probability. Scientists estimate there are about 1,000 NEOs, so NASA is more than halfway to accomplishing its goal.

Reimold notes that this initiative and projects such as the British Taskforce on Potentially Hazardous Near Earth Objects and the Intercontinental Scientific Drilling Program into the Chicxulub crater in Mexico have helped scientists understand the risks and consequences of collisions with asteroids and comets.

The developing world, he said, is slower to catch on, but a movement by astronomers and geoscientists in South Africa to establish a National Working Group to assess NEO impact risk and mitigation is gaining traction.

"On the other hand, the general public in developing countries has a host of other problems than the possibility that a large bolide could wipe out mankind," he said. "If your first concern is to have shelter and food, if HIV/Aids and unemployment are your daily worries, you cannot be expected to be wary of meteorite impact."

More Mitigation Funding?

Writing in Science, Milani says that the scientific community should take on the responsibility to investigate all objects that could potentially impact Earth "down to the size compatible with available technology and with the public perception of acceptable risk."

According to Milani, a reasonable goal would be to detect within the next ten to 20 years 90 percent of the NEOs over 1,000 feet (300 meters) in diameter and 97 percent of those greater than 1 kilometer in diameter.

To accomplish this goal, Milani says that understanding and awareness of the impact risk must be raised amongst the public and the agencies that provide the requisite funding to perform the work.

"If [funds] are provided, the scientists would know how to use them efficiently," he said. "If resources dedicated to this task are not provided, the scientists have difficulties in canceling other worthwhile basic research to make resources available for impact risk assessment."

Reimold said that more money ought to also be made available for research into known and potential impact sites. Currently, he said, only a few impact sites older than 300 million years are known, but that many more should be out there.

"Ongoing detailed geological analysis of known impact structures is a must in order to further improve our knowledge of the impact process and its devastating results," he said.

Robert Jedicke, an asteroid expert with the Institute for Astronomy at the University of Hawaii, said that "it would be nice" if asteroid researchers had more money but that current funding for the NEO impact risk assessment programs is sufficiently supported given the available funding for all scientific research.

"There's only so much money to go around," he said. "So if the pot gets split there's less stew for the rest of the astronomical/scientific community."

NEO Deflection

As NEO researchers continue to search the skies for objects that pose an impact risk, they are also beginning discussions on how to deflect an object on a collision course with Earth.

One of the issues being explored is the interior structure of asteroids. If the interior is weak, for example, an attempt to deflect it with a nuclear warhead (an option under consideration) may simply breakup the asteroid into many smaller and uncontrolled pieces.

Milani writes that such investigations are a valid extension of the NASA and European Space Agency NEO programs and make logical sense: "We cannot justify the effort for discovery unless we can safeguard our planet."

Jedicke said that we are not currently prepared to deflect an incoming asteroid, but that there is no reason to be alarmed because there is little chance that an asteroid even as small as 330 feet (100 meters) will hit Earth within the next 100 years.

"They don't build tornado shelters in Germany. Cities don't buy snowplows in Florida. And there's no pressing need to worry about deflection of incoming NEOs at the moment," he said.